In semiconductor manufacturing processes, the presence of defects inside wafers causes deterioration or impairment of electric characteristics in a semiconductor device as a manufactured article. In manufacturing semiconductor devices, therefore, wafers are inspected for defects at a stage before semiconductor manufacturing or after being subjected to a surface treatment during the manufacturing process. When a wafer having defects is processed as-is, the final semiconductor article becomes a defective one. Therefore, defects must be eliminated beforehand.
Recent years have witnessed ever higher degrees of integration in semiconductor devices and ever finer patterns in the devices and thus the size of wafer defects to be inspected has become smaller. The need for higher defect detection power has increased accordingly. Defect detection includes destructive methods and non-destructive methods. In the former, the wafer is dissolved in an etching solution or is physically abraded to expose, on the surface, defects that are then observed with a microscope or an electron microscope. However, wafers inspected in accordance with the above methods can no longer be used for semiconductor device manufacturing.
Non-destructive inspection methods include electric methods and contact-less inspection methods that utilize light or ultrasonic waves. In electric inspection methods, electrodes are attached to the wafer or probes are made to contact with the wafer. Electric signals are then applied to the wafer and the presence of defects in the wafer is detected on the basis of changes in the electric signals. However, it is difficult to pinpoint thereby the position of the defects. Also, contact with electrodes or the like is required. Such methods cannot be used thus at the manufacturing stage of the article.
In defect detection by ultrasonic waves, ultrasonic waves are applied onto the object to be inspected and the ultrasonic waves reflected by defects are detected by a detector. Internal defects in a material through which light cannot pass, such as metals or the like, can be detected and hence the method is used, for instance, for inspecting package interiors. In terms of detection limits and resolving power, however, the method cannot be used for detecting wafer defects and foreign matter with high resolution.
In inspection methods that utilize light, light scattered by defects or foreign matter is detected by a optical system placed in dark-field or bright-field and position of defect is detected at the same time. For detecting defects inside wafers, lasers, for which silicon is transparent, are used, while visible-light lasers are used for detecting defects in the surface or surface layers.
Defect inspection schemes that utilize light or ultrasonic waves are disclosed in prior art documents such as the following.
Japan Patent Application Laid-open JP, S62-177447, A (Patent Document 1) discloses an ultrasonic damage inspection method for objects to be inspected such as piping or steel, wherein electromagnetic ultrasonic waves are transmitted to the object to be inspected, a laser beam is aimed at the portion of the object to be inspected that is excited by the ultrasonic waves and defects in the object to be inspected, plate thickness and the like are detected on the basis of resulting reflected signals.
Japan Patent Application Laid-open JP, 2001-208729, A (Patent Document 2) discloses a defect detection device for detecting defects, wherein surface elastic waves from an ultrasonic vibrator impinge on an object to be inspected, a laser beam is irradiated onto the surface of the object to be inspected, the resulting reflected light is received, the frequency difference between the laser output light and the reflected light is detected by a signal processing device and vibration speed in the object to be inspected is measured on the basis of that difference.
Japan Patent Application Laid-open JP, 2005-147813, A (Patent Document 3) discloses a method and device for non-destructive inspection of a material, wherein internal defects of an object to be measured are detected by irradiating a pulsed laser beam onto the surface of the object to be measured, to generate elastic waves thereby; irradiating a continuous-emission laser beam for signals, coaxially with the pulsed laser, onto the surface of the object to be measured; and causing reflected light, influenced by the elastic waves and the scattering surface of the object to be measured, to impinge on a laser interferometer, whereby changes in a frequency component are detected.
Japan Patent Application Laid-open JP, 2002-188999, A (Patent Document 4) discloses that a laser beam is irradiated onto an object to be inspected such as a semiconductor wafer or the like; reflected and scattered light from the object to be inspected is detected in a plurality of directions; and the directionality of the reflected and scattered light is detected through comparison of the detection results, thereby foreign matter and defects, such as flaws or the like, in the object to be inspected being detected as well as distinguished therebetween.
Japan Patent Application Laid-open JP, H11-211668, A (Patent Document 5) discloses a defect inspection method wherein a laser beam impinges on a sample to be inspected, the resulting scattered light and the emission light are split into components with a plurality of dissimilar wavelength bands and form images on an imaging device and the nature of the defects is identified on the basis of the obtained plurality of images.
Japan Patent Application Laid-open JP, 2000-216208, A (Patent Document 6) discloses an inspection method in which two pulsed-emission laser beams, set to be at dissimilar incidence angles and have emission timings offset from each other, are irradiated onto the surface of a semiconductor wafer or the like, one of the laser beams being set so as to give rise to scattered light from both particles and pits and the other laser beam being set so that there is less scattered light from pits, wherein particles are distinguished from pits on the basis of the detection results from both types of scattered light.
In the defect inspection methods disclosed in Japan Patent Application Laid-open JP, H10-293101, A (Patent Documents 7) and Japan Patent Application Laid-open JP, H10-293102, A (Patent Document 8), a wavelength λ1 at which reflectance R takes a maximum value and a wavelength λ2 at which reflectance R takes a minimum value, upon a change of the wavelength of a laser beam that impinges on an object to be inspected, are determined beforehand and optical information is obtained at the time at which laser beams of wavelengths λ1, λ2 impinge on the object to be inspected, whereby surface defects are distinguished from defects very near the surface layer of the object to be inspected. Also in this, the laser beams impinge obliquely on the object to be inspected and a total image which shows scattering by defects can be observed in a microscope that is disposed above the object to be inspected.
Japan Patent JP, 3664134, B (Patent Document 9) discloses a method for inspecting a semiconductor wafer surface, wherein a laser beam is irradiated onto and scanned over a wafer surface; light reflected or scattered by the wafer surface is received by a plurality of light-receiving systems having dissimilar light-receiving angles (high angle, low angle) with respect to incident light; and differences between standard reduced particle sizes on the basis of ratios of the light intensities received by the plurality of light-receiving systems are obtained, so as to determine the character and type of the defects.
Japan Patent Application Laid-open JP, 2008-8740, A (Patent Document 10) by the present inventors discloses a method and apparatus in which a laser beam is irradiated onto a wafer surface in a state where ultrasonic waves are being applied onto the wafer and in a state where ultrasonic waves are not applied and the change of intensity of light scattered by cavity defects, from before to after application of ultrasonic waves, is detected by a light-receiving means disposed in a cross-Nicol arrangement with respect to a polarizer, so that foreign matter is determined on the basis of changes in the intensities of the scattered light.
In Patent Documents 1 and 2, internal cavity defects cannot be detected with high resolution. In Patent Document 3, the presence or absence of internal defects can be detected but the influence on a scattering surface of the material surface, caused by ultrasonic waves, is detected in the form of signal light. This is appropriate for non-destructive inspection of concrete structures but not for high-resolution inspection of internal defects in semiconductor wafers or the like.
In Patent Documents 4 and 5, the nature of defects is identified on the basis of a relationship between directionalities of reflected or scattered light and wavelength bands. This approach, however, is not appropriate for high-precision detection of internal defects. In Patent Document 6, two pulsed laser beams are irradiated at timings offset from each other, hence the composition and control mechanisms involved are complex. Also, although surface defects such as particles and pits can be detected thereby, the method is not appropriate for detecting internal cavity defects.
In Patent Documents 7 and 8, surface defects and internal defects are distinguished on the basis of wavelength differences. However, it is not possible to determine whether the defects are internal cavity defects or not.
In Patent Document 9, the type and character of wafer surface defects are determined according to a combination of numerical values of standard reduced particle size of scattering elements, on the basis of scattered light intensity ratios at dissimilar light-receiving angles. However, cavity defects inside the wafer cannot be determined thereby.
In case of detecting foreign matter on a surface layer with the method disclosed in Patent Document 10, surface shift caused by ultrasonic waves may give rise to changes in scattering intensity, which in turn may result in misdetection. Also, only P-polarized or S-polarized light is detected, hence defects cannot be classified with sufficient certainty, which is problematic.